The concentration of amine ends (usually expressed as meq/kg polymer) in polyamide polymers affects the affinity of shaped articles made of these polymers for certain dyestuffs. The amine-end concentration determines the dye capacity of polyamide materials and variations in amine-end concentration will cause nonuniformities in the depth of shade after dyeing. Light-dye polyamide yarn has about 10.+-.5 meq/kg amine ends, normal mid-dye polymer about 40.+-.5 meq/kg amine ends, and deep-dye polymer about 70.+-.5 meq/kg amine ends. The dyeability of a polyamide may be measured in dye units, as described in the ABB dye test given below. A value of 180 units is used to adjust and normalize sample dyeability to a known base. Control is typically 180.+-.13 dye units for bulk continuous filament polyamide fiber and future trade requirements will demand control within about .+-.6 dye units. A change in amine-end concentration of 1 meq/kg will result in a change in fiber dyeability of about 12 dye units. Hence, in order to control dyeability to within .+-.6 dye units, amine-end groups must be controlled to within .+-.0.5 meq/kg.
In a conventional batch polymerization process for the production of polyamide polymers, a polyamide precursor salt is concentrated in a batch evaporator, the concentrated salt polymerized in a batch autoclave, extruded into a solid ribbon, and chipped into pellets or granules commonly referred to as polymer flake. The polymer flake is melted in an extruder and extruded into various shapes depending on the desired end use. If the properties of the extruded material are not within specifications, the composition of the next batch of polymer can be appropriately adjusted. For example, in the case of polyamide fiber dyeability, the amount of diamine or diacid can be adjusted in the autoclave during the preparation of the base polymer flake to control the amine-end concentration and bring the dyeability within specified limits. However, during the lag time that occurs between detection of the deviation and adjustment of the composition of the salt solution used to prepare the polymer flake (which can be on the order of 8-24 hours), large quantities of fiber may be produced with out-of-limits dyeability, resulting in either yield loss or increased dye variability. In the case where the polyamide base flake is not manufactured in-house, but rather shipped from a supplier at another location, a new shipment must be ordered if the properties are not within the desired limits which can involve lag times of weeks to months.
U.S. Ser. No. 07/425,388 describes a process whereby batch-produced polyamide dyeability is modified by injecting a diamine into a low pressure region of a screw melter extruder to increase the total amine end-group concentration. This permits quick response to deviations in polyamide dyeability. However, it requires a special injection system to accomplish the diamine addition.